I have the Sigma 30mm lens and I like it. Center sharpness is great at all apertures and corners don't matter that much in most of my images. With landscape photos you can see the softness there if you look 100 % size but it's only very soft in the area of ~20-30 nearest pixels to the corner. Therefore it can be cropped away if it bothers, but usually you won't even notice it if not looked at 100 %.
I uploaded a few photos taken with this lens at different apertures to Flickr:
Sigma 30mm f1.4 - a set on Flickr